Betelgeuse is a truly massive star. It is located about 725 light-years from Earth, and its mass of about 17 times the mass of the Sun and its radius of about 800 times the radius of the Sun make it one of the largest stars in our galactic neighborhood. This star is easily distinguishable with the naked eye in the winter night sky, as it shines with an intense orange or reddish hue, located on the left shoulder of the warrior that forms the constellation of Orion .
This color is not an artifact or distortion caused by the atmosphere, but is the actual color of the star . Betelgeuse is what is known as a red supergiant , a star that, after exhausting all its fuel, has grown in size several hundred times , in what should be the final stages of its evolution until it explodes within a relatively short time in the form of supernova . The star is currently just over 8 million years old and we are convinced that in the next few hundred thousand years it will end its days with an explosion. However, Betelgeuse still has a lot to tell us , even before he died.
At the end of 2019 , this star was detected to be losing brightness . All the alarms went off, as it was thought that this could be the start of the supernova explosion that would end his life. However, in the last two and a half years the star has been recovering and returning to brightness levels similar to those of then . Recent research in which several observatories have collaborated, both on the ground and in orbit around the planet, the main one being the Hubble telescope , have found an explanation for this loss of brightness.
Apparently in 2019 (or rather 725 years before 2019) Betelgeuse lost a significant part of its surface in what has been dubbed a surface mass ejection . These would be similar to coronal mass ejections often seen on the Sun, during which our star expels large amounts of matter from the solar corona , the sun’s outermost layer. However, the similarities between the two phenomena end there, because in the case of Betelgeuse the ejection has taken place from the star’s own surface , from its photosphere and because some four hundred billion times more mass has been ejected than in the equivalent sun .
It is the first time that we observe such an event, from which the star has not yet fully recovered, as we continue to perceive certain vibrations inside Betelgeuse , due to how they affect its surface. However, in the two years since we observed the dimming of the red supergiant, the possibility that this event is an indicator of an imminent explosion has been losing support . So we believe this could be just one more of the many processes that take place in stars of this type.
On the other hand, we now understand that the dimming was not due to a change in the surface of Betelgeuse itself after the mass ejection, but because this large amount of ejected gas , which would accumulate a mass several times that of our own Moon, would have formed a cloud of gas that, when cooled, would have hidden part of the star’s surface as we see it from Earth. That is, ejections like this have probably occurred before, but since the resulting cloudlet has not been located between Earth and Betelgeuse, we have been unable to detect them . There is hope that the James Webb Space Telescope, which began observing the universe just a few months ago, will be able to observe this cloud in the infrared , giving us more information about this whole process.
We believe that the ejection took place as a result of a process similar to that which causes volcanic eruptions on Earth. A pocket of especially hot plasma , which could reach sizes of more than a million kilometers, rose to the surface of Betelgeuse, where it ended up erupting and expelling large amounts of material. However, since Betelgeuse is so large, these events only cause a small disturbance to its surface , and the mass lost is only a tiny percentage of the total mass of the star.
What this ejection did affect is the cycle that the star had been showing for centuries, lasting about 400 days , according to which it was periodically gaining and losing brightness. This cycle was no longer observed after the mass ejection, although it is believed that it could reappear once the star has returned to normal . We have been able to observe all of this thanks to the relative proximity of Betelgeuse and its enormous size, and thanks of course to the incredible technology that we have developed in recent decades. In 1996 , Betelgeuse was precisely the first star from which we could distinguish superficial characteristics , beyond the Sun. Until then, all stars had been nothing more than tiny points without size, even when we used the most powerful telescopes. Being able to study their surface with more precision allows us to know them in greater detail.
References:
Andrea K. Dupree et al, 2022, The Great Dimming of Betelgeuse: a Surface Mass Ejection (SME) and its Consequences, The Astrophysical Journal, https://doi.org/10.48550/arXiv.2208.01676
